Magnesium Based MMCs Reinforced with C-Fibers

Hajo Dieringa, Norbert Hort and Karl Ulrich Kainer

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Posted: September 2005

Topics Covered

Abstract

Keywords

Introduction

Manufacturing of MMCs

Experimental

Results and Discussion

Tensile Testing

Creep Testing

Thermal Expansion

Microstructure

Conclusion

References

Contact Details

Abstract

Metal Matrix Composite’s (MMCs) have gained increasing interest in the last decade (and in recent decades), particularly in automotive applications, but also in general transportation there is a need to develop high strength, creep resistant lightweight components with controlled coefficient of thermal expansion (CTE).  Aluminum based MMCs are already in use, for example as fiber reinforced pistons or particle reinforced brake disks.  Because of poor high temperature properties of commercial magnesium alloys and an increasing demand for weight saving materials, research and development of magnesium based MMCs is still necessary.  In this paper, latest developments in carbon fiber reinforced magnesium alloys are presented.  Carbon fiber reinforced high temperature resistant magnesium MMCs manufactured by squeeze casting process were investigated.  Due to the manufacturing process - infiltration of prefabricated carbon-fiber performs.  The MMCs show anisotropic mechanical behavior because fibers show a planar random distribution.  The preform used influences the effect of reinforcements on the properties of the MMC.  The composites investigated are carbon fiber reinforced AS41, AE42, AZ91Ca, AM50, AM50Nd and AZ31 magnesium alloys.  The creep properties were determined in the temperature range of 150-300°C.  The stress was varied between 30 and 90 MPa.  The results reveal that the use of short carbon fibers enables the creep resistance to be increased markedly.

Keywords

C-Fibers, Magnesium Alloy, AZ31, AE42, AM50, AS41, Creep, MMC

Introduction

In lightweight construction, magnesium alloys are of interest due to their relatively low density and high specific strength.  However, due to increasing demands especially in high temperature performance of light materials, development of new low density and high temperature resistant materials is required.  Strength, creep resistance and thermal expansion are the most important properties for judgment of materials suitability in the transportation industry.  High temperature creep properties of magnesium alloys can be increased by alloy development in a significant way.  Examples include QE (silver and rare earths) and WE (yttrium and rare earths) magnesium alloys.  In the past, thorium was also an alloying element for magnesium alloys, but due to radioactive isotopes of thorium it is no longer in use.  Importantly the high costs of these alloying elements are one of the main disadvantages of these alloys.  In addition, the coefficient of thermal expansion (CTE) cannot be influenced by alloying elements.  An alternative can be magnesium based Metal Matrix Composite’s (MMCs).  It has already been shown that fiber and/or particle reinforcement improves high temperature properties of magnesium based MMCs compared to their matrix alloys [1].  But also the disadvantage of higher production costs due to more complicated manufacturing processes has to be taken into account.  Only the use of cheap materials - both the alloy and the reinforcement – in relation to cost effective production processes for manufacturing of magnesium based MMCs can introduce this class of low density materials into the market.  In this paper, production and materials properties of some MMCs based on common magnesium alloys such as AZ91, AE42, AZ91Ca, AM50, AM50Nd and AS41 as the matrix alloy and with reinforcement of short carbon fibers are presented.  MMCs are manufactured by the direct squeeze cast process where a prefabricated preform made of C-fibers or of C-fibers and Si-particles is infiltrated under pressure with magnesium alloys.  Creep properties, strength and thermal expansion behavior were investigated and results are accompanied by optical and electron microscopy.

Manufacturing of MMCs

All magnesium based MMCs were produced by a direct squeeze casting process [2,3,4].  This process is characterized by slow, laminar filling of the die, high pressure during infiltration and solidification, and compared to die casting, a slow cooling and solidification, which leads to a relatively long contact between the melt and reinforcement.  Furthermore, the high pressure during solidification reduces the porosity of the component.  Compared to die cast components with a porosity of about 3%, squeeze cast parts show only about 0.5% porosity.  Figure 1 shows a sketch of a squeeze casting tool.  Preforms were preheated to 400°C in order to avoid solidification during the infiltration process.  The melt was superheated to between 700°C and 720°C and poured over the preform.  The vertical stamp squeezes the melt with a pressure of approximately 60 MPa into the preform and the solidification takes approximately one minute.  Preforms for AS41 based MMCs contain 18 vol.% PAN-based Sigrafil^®C-fibers.  Investigations with AZ91 + Ca as matrix were performed with different amounts of Sigrafil^®C-fibers in the preform.  The preforms for the hybrid composites were made of 7 vol% Sigrafil^®C-fibers with a length of about 180 µm and a diameter of about 7 µm and of 4 vol% Si particles with a diameter of ≤45 µm.  The share of Silicon-based binder that stabilizes the preform amounts to 4 wt%.  In all cases, fibers in the preform show a planar isotropic distribution.  Table 1 shows some properties of the materials and Table 2, the chemical composition of the alloys.

Table 1. Properties of the materials employed.

Table 2. Nominal chemical composition of the Mg based alloys employed [wt%].

Experimental

Tensile tests were performed at room temperature as well as at 150°C, 200°C and 250°C.  Creep properties of magnesium based MMCs at constant temperature and constant load were evaluated at temperatures between 150°C and 300°C and loads between 30 MPa and 90 MPa in tensile creep tests using round specimens.  Thermal expansion was measured with a dilatometer DIL402C from Netzsch.  Cylindrical specimens 25 mm in length and 5 mm in diameter were investigated with the one-dimensional dilatometer in a range of room temperature (RT) – 300°C.  Three temperature-cycles with a heating and cooling rate of 5 K/min were performed.

Results and Discussion

Tensile Testing

In order to evaluate how the fiber distribution effects the strength of the C-fiber containing MMCs, tensile tests with AE42 based material were performed at RT, 150°C, 200°C and 250°C.  Figure 2 shows the ultimate tensile strength (UTS) of specimens tested parallel and perpendicular to the fiber-plane.  At 200°C, AE42 has a UTS of 95 MPa.   The reinforced MMC perpendicular to the fiber-plane also has a UTS of 95 MPa but the UTS of the MMC parallel to the fiber plane is 155 MPa.  The carbon fibers do not increase the strength at elevated temperatures, when tests are performed perpendicular to the fiber-plane.  The tensile strength (UTS) only increases when the test direction is in the fiber-plane (longitudinal), in this case, by more than 60%.  In accordance to the rule of mixtures (ROM) [7], the strength of the MMC is improved only in fiber direction.  Hybrid composites based on AZ31, AM50 and AM50+Nd were tested only at 200°C. Table 3 shows the UTS in comparison with standard alloys.

Table 3. UTS of hybrid MMCs at 200°C compared with standard alloys.

Creep Testing

Tensile creep tests with constant stress at temperatures of 150°C and 200°C were performed at 50, 60 and 70 MPa with AZ91+Ca based MMCs until break occurred.  Table 4 shows minimum creep rates in those tests.  Stress dependence of the minimum creep rate is given by:

Table 4. Results of creep tests with AZ91Ca based MMCs

In order to evaluate the stress exponent n for 200°C a log/log-plot of minimum creep rate as a function of load is shown in Figure 3. Similar creep tests were performed on the AS41 based MMCs.  At temperatures between 150°C and 300°C and loads between 30 and 90 MPa tensile creep was investigated [9].  Evaluation of the stress exponent n, as shown in Figure 4, leads to very similar results.  The stress exponent at 200°C is higher than for the AZ91Ca based MMCs.

Hybrid composites reinforced with carbon fibers and silicon particles were tested at 200°C and 60 MPa load. Creep curves are shown in Figure 5. The minimum creep rates for the AZ31 based MMC is 4.6x10^-9 s^-1, for the AM50 based MMC is 7.5x10^-8 s^-1 and for the AM50Nd based it is 1.1x10^-7 s^-1.

Thermal Expansion

The coefficient of thermal expansion (CTE) was measured from hybrid MMCs as well as from AS41 based, and AZ91 + Ca based MMCs.  After the first cycle the specimen shows a dimensional change in length because of the release of internal stresses which are generated during the squeeze casting process.  The coefficient of thermal expansion in the transverse direction and in the longitudinal direction were calculated and are given in Table 5.  Whereas the CTE in the direction transverse to the fiber-plane is hardly reduced compared to the matrix alloy, in the longitudinal direction there is a decrease of approximately 20%.  Also in accordance with the rule of mixtures, the low CTE of the carbon fibers reduces the CTE of the MMC only in the fiber direction but not in the transverse direction.

Table 5. CTE of hybrid and fiber reinforced MMCs.

Microstructure

The fibers in the preform show planar isotropy.  Using the squeeze casting process, the direction of infiltration is normal to the fiber plane in the preform.  This leads to an anisotropy of mechanical properties in the squeeze cast components.  Due to dimensional limitations of the parts, specimens were prepared for tensile tests and creep from the fiber direction plane (longitudinal).  Specimens for dilatometric investigations were tested in both longitudinal and perpendicular directions.  A micrograph of a AZ31 based hybrid MMC (Figure 6) shows dark carbon fibers and two types of Mg₂Si-precipitates.  The “Chinese script” type is the reaction product from the SiO₂-binder that stabilizes the preform.  The second type of Mg₂Si-precipitation is formed from the massive silicon in the preform.  Figure 7 shows the carbon fiber reinforced AS41 based MMC.  Unreinforced areas are recognizable as well as areas where fiber clustering occurs.  The reasons for inhomogeneities in mechanical properties are obvious.

Microstructural investigation after creep testing at 200°C and 50 MPa shows fiber cracking (see Figure 8).  Figures 9 and 10 both show SEM micrographs of fiber cracking after creep testing at 200°C.  The load varies from 50 MPa in Figure 9 to 60 MPa in Figure 10.  In both cases, fibers are surrounded by a precipitation layer.  Whereas the bonding between fiber and layer is good in Figure 9 there is debonding in Figure 10.  This damage leads to a reduced lifespan during creep.  Creep testing at 50 MPa lasts 840 h whereas the test at 60 MPa lasted 190 h.

Conclusion

Fiber and hybrid reinforced magnesium alloys show an increase in strength, creep resistance and a reduced thermal expansion.  Due to the planar isotropic distribution of fibers in the MMC, strength is in accordance with the rule of mixtures and only improved in tests performed along the fiber-plane.  The reduction in thermal expansion of approximately 20% along the fiber-plane for the same reasons brings the CTE values close to those of aluminum alloys.  Creep resistance is improved by up to two orders of magnitude compared to the matrix alloys.  Carbon fiber reinforced magnesium alloys show good mechanical properties but still have disadvantages compared to aluminum alloys, due to higher production costs.

References

1.       V. Sklenicka, M. Pahutova, K. Kucharova, M. Svoboda and T. J. Langdon, “Creep of Reinforced and Unreinforced AZ91 Magnesium Alloy”, Key Eng. Materials, 171 (2000) 593-600.

2.       K. U. Kainer and E. Böhm, “Squeeze Casting of Magnesium Alloys”, VDI Berichte, 1235 (1995) 117-125.

3.       K. U. Kainer and B.L. Mordike, “Herstellung und Eigenschaften von Kurzfaserverstärkten Magnesium Legierungen“, Metall., 44 (1990) 438-443.

4.       G. A. Chadwick, “Squeeze Casting of MMCs Using Short Fiber Preforms”, Mater. Sci. Eng., 135A (1991) 23-28.

5.       SGL Carbon, data sheet Sigrafil C.

6.       R. J. LaBotz and D. R. Mason, “The Thermal Conductivities of Mg₂Si and Mg₂Ge”, J. Electrochem. Soc., 110 (1963) 121.

7.       K. K. Chawla, “Composite Materials”, ISBN 0-387-98409-7, Springer p.193.

8.       ASM Specialty Handbook, “Magnesium and Magnesium Alloys”, M.M. Avedesian and H. Baker, ASM International, ASM International, (1999) 177, 0-87170-657-1.

9.       B. Sommer, Ph.D Thesis “Untersuchung Zum Kriechverhalten der Kohlenstoffaserverstärkten Magnesiumlegierung AS41“, TU Clauthal Germany, (2000).

Contact Details

Hajo Dieringa

GKSS Research Center,

Center for Magnesium Technology

Max-Planck-Str.1

21502 Geesthacht

Germany

E-mail: [email protected]

Norbert Hort

GKSS Research Center,

Center for Magnesium Technology

Max-Planck-Str.1

21502 Geesthacht

Germany

Karl Ulrich Kainer

GKSS Research Center,

Center for Magnesium Technology

Max-Planck-Str.1

21502 Geesthacht

Germany

This paper was also published in print form in “Advances in Technology of Materials and Materials Processing”, 6[2] (2004) 136-141.